Influence of external vibrations on bubble coalescence time at water and oil surfaces—Experiments and modelling

Abstract

We report results of the experiments and numerical simulations on kinetics of collision, bouncing and coalescence of air bubbles at free surface of pure liquids of different viscosities. The bubble collision and bouncing courses on the stagnant and vibrating (with controlled acceleration), water/air and silicone oil/air interfaces are compared. For stagnant interfaces the coalescence time (tc) was found to be the bubble radius (Rb) dependent. For larger bubbles the tc was longer. This was caused by higher impact velocity resulting in an increased bubble deformation and higher tendency of the bubble to rebound from the liquid/gas interface. At oscillating liquid/gas interfaces with proper vibration frequency and amplitude the bubble coalescence time can be prolonged significantly as a consequence of prolongation of the bubble bouncing time. This was due to the fact that the energy dissipated during the bubble collision and bouncing was re-supplied via interface vibrations with a properly adjusted acceleration. The analysis of the bubble deformation showed that this effect is related to the degree of the bubble deformation, which determined size of the liquid film formed at the interface. The bubble was bouncing when radius of the liquid film formed was large enough to prevent the draining film to reach a critical thickness of rupture during the collision time. Moreover, it was found that higher acceleration should be applied to prolong the coalescence time (tc) of the bubbles having higher Laplace pressure. The results obtained prove that mechanism of the bubble bouncing from various interfaces depends on interrelation between rates of two simultaneously going processes: (i) exchange between kinetic and surface energies of the system, and (ii) drainage of the liquid film separating the interacting interfaces.